Why is Mewtwo a bad matchup for Fox?

So I've been learning how to counter Foxes, for obvious reasons. As you can see by my flair, my main is Mewtwo. I've been looking up his bad matchups in order to learn those characters and beat him. As a pleasant surprise, I found out that my main is considered by many to be Fox's worst matchup. However, as Mewtwo, I still tend to struggle against Foxes. Can someone please tell me what tactics I should be using to beat him?

EDIT: I just put a Level 9 Mewtwo CPU up against a Level 1 Fox CPU, and the Level 9 Mewtwo lost.